This invention relates to a carbon dioxide miscible displacement process utilizing a special transition zone. More specifically, prior to carbon dioxide oil miscible displacement of a subsurface oil-bearings reservoir, there is created a transition zone of demetanized oil enriched with propane with or without heavier hydrocarbons. The transition zone is created by one or more propane huff and puff sequences.
The production of oil is enhanced by various displacement techniques which are generally classified as miscible and immiscible and which may be conducted at any time during an oil recovery program. In these displacement techniques, the force of an injected fluid propels oil within the formation toward a producing well or horizon. Two types of immiscible displacement processes involve flooding the reservoir with carbonated water or with carbon dioxide at a pressure below the miscible pressure. Three types of miscible displacement processes are the miscible slug process, the enriched gas or condensing gas process, and the high pressure gas process. A extremely large variety of miscible recovery processes have been disclosed. But, in general, these processes may be considered as a variation of or a combination of one of the three basic miscible processes. Quasi miscible chemical flooding processes have also been developed. Some of these chemical processes utilize foam and others utilize surfactants which greatly reduce the surface tension between the displacing fluid and the in place oil.
This invention relates to the use of carbon dioxide to miscibly displaced oil from a reservoir. Carbon dioxide generally builds miscibility in the same way that the high pressure gas process builds miscibility, but the carbon dioxide does this at a lower pressure. Because carbon dioxide does not have first contact miscibility with the oil and must build miscibilty by a series of enrichments and exchanges with the in place oil, it has been proposed to inject a bank or slug of propane solvent ahead of the carbon dioxide. Propane solvent slug sizes vary from a few hundredths of the reservoir pore volume to ten to twelve percent of the pore volume. The solvent is injected as a relatively narrow transitional displacing phase between oil and the carbon dioxide. Materials other than propane may be employed to provide a combination slug. Water and other additives have also been combined or alternately injected with the propane solvent to partially influence unit displacement of the slug of solvent. When the propane solvent is mixed with water, the process is still a miscible flood process in that the propane acts as a distinct phase of the mixture. In addition to economic factors, there are serious problems involved in the propane miscible slug processes. Unless an expensively large solvent slug is used, it is difficult to form and maintain a uniform flood front of sufficient thickness and breath to prevent loss or depletion of the band of solvent. If the solvent band is broken or depleted, miscibility is lost and an immiscible drive results. The miscibility cannot be reestablished unless an additonal solvent slug is injected. Because of these problems, it has been proposed to inject enriched carbon dioxide slugs. For example it has been proposed to add propane and propane plus hydrocarbons to the carbon dioxide to enhance miscibility with the oil.
As previously stated, in the carbon dioxide miscible process, miscibility is built only by multiple contacts between the oil and the displacing fluid unless sufficient hydrocarbon solvent is used to create first contact miscibility. the injected carbon dixiode fluid enriches the oil and the oil enriches the injected fluid until a transition zone of miscibility is established. The leading edge of this zone is substantially miscible with the oil and is very much like oil, except possibly for some relatively small precipitated heavy oil phases. The trailing edge of the zone is miscible with the carbon dioxide and is very much like the carbon dioxide fluid. Inside the transition zone, all contiguous fluids are miscible at their leading and trailing edges. In the carbon dioxide miscible displacement processes, the process is pressure dependent. In many cases, carbon dioxide miscible displacement processes require a pressure greater than desirable for the reservoir. This may be caused by the properties of the reservoir oil or by the nature of the formation. It is to be especially noted that methane interferes with miscibility between oil and carbon dioxide. In addition, it is often impratical or undesirably costly to achieve and maintain the relatively high pressures needed for miscibility. In many reservoirs, these relatively high pressures also adversely affect sweep efficiency.
In the carbon dioxide displacing process, depending on the reservoir, one or more problems may result from such factors as gravity override or segregation, viscous fingering, reservoir stratification and the like. In horizontal displacements, such factors affect both the horizontal areal sweep and the vertical sweep. These factors especially influence carbon dioxide miscible displacement processes, including a propane slug followed by carbon dioxide.
It is standard practice to employ laboratory testing and reservoir data to establish and start a reservoir program; therefore, the principles involved in these processes are well known. But tests concerning carbon dioxide displacements are frequently subject to uncertainties especially as to the minimum pressure required to create a miscible zone and to problems involving deasphalting or leaving some high molecular weight components in the test core. It, therefore, would be advantageous to provide a carbon dioxide miscible process wherein less propane is required and wherein miscibility can be achieved more confidently at a given pressure. The pressure may be kept as low as practical, thereby allowing for more confidentally designing the process, overcoming a part of the gravity override and viscous fingering problems encountered in miscible displacement processes, and using less carbon dioxide gas because at a lower pressure the gas is less dense and thereby occupies more formation volume per standard cubic foot of gas with the same oil production.